Cast Iron

1: Break off some of the little shiny specks.

2: See if those specks will stick to a strong magnet. (If not, that's a bad sign.)

3: Do one of those at-home test kits on only the specks.

If it didn't stick to the magnet and the test kit says positive for lead, then you know you're not getting a false positive on iron -- if it was iron or anything ferrous, it would have stuck to the magnet. That doesn't absolutely guarantee the accuracy of the test kit, but it would eliminate one common false positive.

If you can separate a sufficiently large speck, you could also do a hardness test on it: what materials is it able to scratch, and what materials rub material off of it? Lead is very much on the softer side, so if the specks are easily deformed and abraded -- especially when tested against things like aluminum -- that's another bad sign that it might be lead.

And, at any rate, I'd be leery of it if the specks don't stick to a magnet. The list of non-magnetic, silver-colored metals you want in contact with your food is pretty short, while the list of ones you don't want in your food is pretty long. Lead is probably the most likely and the worst offender on the list, but there are other possibilities that people might have been messing around with ... and most of those other possibilities aren't great to have in your food, either.

I have a data point of size ~5, when a friend purchased a bona fide lump of lead in order to be the control for his lead test strips. And he basically used the test strips in pairs: one on the object in question (eg cast iron pan, ham radio, etc) and then the other against that lump of lead. There have not been any false-negatives from the lump.

Unfortunately that's a bit outside of my area of expertise, I never really see false negatives being discussed when this comes up, I'm sure they happen, not many tests are 100% foolproof either way.

Most lead tests are really designed for paint, so using them on a pan is kind of outside of what they're made for to begin with and I'd be a little skeptical of the results either way.

My understanding, and it may be wrong, my knowledge on the tests comes mostly from places like this where people are cast iron geeks more than chemists, are that the tests are almost more of a test for the presence of metal in general not so much specifically lead, so you get a lot more false positives because of course there's more metal in a metal pan than you'd expect there to be in any kind of paint, and it's better to err on the side of caution and give a false positive instead of a false negative.

Again, that's secondhand knowledge from people who I don't have a ton of confidence in having their facts straight to begin with, so take that with a huge grain of salt.